This invention relates to uninterruptible power supply (UPS) systems. In particular, the present invention relates to UPS systems that provide electric power to critical loads when primary power supplies fail, or when deterioration occurs in the power being supplied to the end user.
UPS systems are widely used to assure that, when a primary power supply fails due to equipment malfunction, downed lines or other reasons, electric power continues to be supplied to critical loads, such as hospital operating room equipment, computer systems and computerized manufacturing equipment. UPS systems avoid equipment failures, costly downtime and equipment damage, as well as providing necessary power that otherwise would not be available.
UPS systems traditionally take two basic forms, inverter-based and rotary. A typical inverter-based UPS system has a utility-powered rectifier connected to a DC bus which charges a string of chemical storage batteries. When the primary power supply fails, electronic circuitry converts direct current (DC) from the batteries into alternating current (AC) which operates the critical load. In an off-line or line-interactive UPS, this AC output is used to power the critical load only whet power is unavailable from the primary power supply. In a double conversion UPS (where utility power is converted from AC to DC and back to AC), the AC output provides power to the critical load at all times.
A typical rotary UPS uses a motor which drives a generator. The generator supplies alternating current to the critical load at all times. The motor is typically a DC motor that is driven during normal operations by rectified DC from the primary power supply, and driven during primary power supply interruptions by a battery string. During very brief power interruptions, the rotational momentum of the motor and generator can supply power to the critical load.
For inverter-based and rotary UPS systems, flywheel systems are available as a clean and reliable alternative or complement to chemical batteries. Such flywheel systems include a flywheel connected to an electrical machine which can operate as a motor and as a generator. For example, U.S. Pat. No. 5,731,645 describes flywheel systems that provide backup power to the load in UPS systems. The electrical machine is powered by the DC buss to operate as a motor when acceptable power is being received from the primary power supply. When power from the primary power supply fails, the electrical machine is rotated by the kinetic energy of the flywheel, and it acts as a generator to supply power to the DC buss.
Large UPS systems in the range of 20 KW to 2 MW often use prime movers (fuelburning engines) to drive backup generators during prolonged power outages. The prime movers are costly and complicated, and they require extensive ongoing maintenance. The engines may fail to start, resulting in loss of power to the critical load. In some localities, ordinances limit the running time or the number of starts per year of the engines for backup generators, which limits testing and overall usefulness of such systems.
Energy storage systems currently used to provide power to a DC buss are expensive and complicated. In battery energy storage systems, there is a risk that undetected battery damage or corrosion of battery terminals will result in a failure to deliver power when needed. Batteries have a limited life and they require expensive ventilation, drainage, air conditioning and frequent maintenance. Flywheel energy storage systems, while avoiding most disadvantages of batteries, are expensive since they are mechanically complex and they require complicated power electronics.
In some existing systems, power from the primary power supply is rectified and transmitted to a DC buss, converted to low frequency AC by a converter, and used to power a critical load. The associated UPS systems have a high speed gas turbine and a backup generator driven by the turbine. The backup generator is a brushless permanent magnet alternator which generates high frequency AC which is rectified and transmitted to the DC buss. The DC output buss provides power to an inverter, and the inverter converts the DC to a low frequency AC which powers the critical load. When the power from the primary power supply is present, the turbine rotor is stationary. When a brief outage occurs, a battery string supports the DC buss. When there is an extended power failure, a battery is connected to the generator which then, acting as a motor, brings the turbine rotor up to speed. When a predetermined minimum speed is attained, fuel is supplied to the gas turbine to sustain the rotation of the turbine, and power from the generator is supplied to the DC buss. Such systems are expensive and complicated compared to the present invention because they require a separate energy storage system.
A primary object of the present invention is to provide a UPS which is less complicated and less expensive than existing battery/generator/turbine UPS systems of the type described above.
Another object of the present invention is to provide UPS systems that include a gas turbine having minimal losses during idle periods.
A further object of the present invention is to provide UPS systems that include a gas turbine in which compressor thrust is significantly reduced to improve the life of the bearings.
A still further object of the present invention is to provide UPS systems that include a gas turbine configured with magnetic rotor weight unloading to lower system costs and improve system performance.
In one respect, the method of the invention is performed by supplying electric power from said primary power source to said motor means to rotate said turbine rotor which stores kinetic energy as rotational momentum; and, operating said apparatus in an emergency mode in which said machine rotor is rotated by rotational momentum of said turbine rotor to supply electric power to a critical load.
Preferably, the turbine rotor includes a plurality of parallel discs which are separated by spaces, and the method includes the step of introducing motive fluid from the fluid supply into peripheral regions of said spaces to rotate said turbine rotor. The motive fluid may be provided by many means, including but not limited to: (1) burning a fuel to produce an exhaust gas, and using said exhaust gas as said motive fluid (2) providing a source of compressed gas, and using said compressed gas as said motive fluid, or (3) boiling water to produce steam, and using said steam as said motive fluid.
According to the preferred method, the primary power source is disconnected from the critical load when the primary power source fails. The electric power supplied to the critical load may be direct current or alternating current.
In another respect, the method of the invention is performed by supplying electric power to a rotary electrical machine which is operating in a motor mode, while transmitting rotational motion from said electrical machine to a rotor of a turbine whereby said rotor stores kinetic energy in the form of rotational momentum; transmitting said rotational momentum of said rotor to said electrical machine when the primary power source fails; and, operating said electrical machine in a generator mode to supply electric power to said critical load. Preferably, a flow of motive fluid to said rotor is directed to the rotor to prevent its rotational deceleration.
In yet another respect, the method of the invention includes the steps of (1) operating an apparatus in a non-emergency mode in which power is supplied from the primary power supply to said electrical machine, said fluid supply is inactive, and said electrical machine rotates said turbine rotor which stores kinetic energy as rotational momentum; and, (2) activating said fluid supply to direct motive fluid to the turbine rotor to sustain rotation of the turbine rotor and the electrical machine rotor to generate electric power which is supplied to the critical load. Preferably, after the primary power supply has failed and before the fluid supply is activated, the apparatus is operated in a TRANSITIONAL mode in which rotational momentum, of said turbine rotor rotates said electrical machine rotor to generate electric power which is supplied to the critical load.
When the electrical machine has a motor unit and a generator unit which have separate windings from each other, and the apparatus is operating in its non-emergency mode, said motor unit drives said generator unit, and said generator unit generates electric power for the critical load. When said electrical machine includes a dual purpose unit which operates as a motor at some times and as a generator at other times, said unit is operated as a motor when the apparatus is in its non-emergency mode and it is operated as a generator when the apparatus is in its emergency mode.
In another preferred embodiment, air drag on the inactive turbine is reduced during idle periods by sealing the turbine housing and either evacuating it, or purging the housing with a light gas, such as helium. In this configuration, solenoid valves, for example, could be used to admit working-fluid and allow exhaust gases to escape once the turbine was activated. In addition, if a shaft-mounted compressor is included, the compressor could be kept in INACTIVE mode while the turbine was inactive to further reduce losses. When working fluid is introduced into the turbine, air would also be introduced into the compressor, so that both components enter ACTIVE mode. Persons skilled in the art will appreciate that it may be advantageous to keep a portion of the housing, for example, the electric motor/generator section, evacuated or purged with a light gas even while in ACTIVE mode.
Other preferred embodiments of the present invention may include, for example, the use of magnetic bearings to unload the weight of the turbine (this would also permit the use of relatively inexpensive grease-lubricated bearings); a power turbine configured in two parts to cancel thrust load while routing exhaust to the center of the turbine, thereby reducing shaft temperature at the bearings; a two-part power turbine configured in unequal parts (where each part provides a different amount of thrust load), so that the extra thrust load of one part cancels out the thrust load of the compressor; a variable housing spacing around the compressor (controlled by, for example, a magnetic bearing) that increases housing clearance during periods of inactivity for reduced drag; the compressor may be used as a vacuum pump to evacuate the turbine housing as described above.